The invention relates to a procedure and tools for conducting a robust, quantitative measurement of surface cleanliness for field installation, factory fabrication, and assembly of mechanical systems such as power plant equipment. More specifically, the invention provides a quantification of the qualitative white glove test often used by cleanliness inspectors.
Surface particulate contamination is a well-known source of mechanical system failures. Particulate contamination can cause abrasion at the interface between moving parts, contamination of fluids flowing through the system and erosion of structures in high velocity fluid flow path(s) and/or create deposits that either reduce desired flows or insulate against desired heat transfer. While the provision of filters and the like can control the flow of particulate contaminates into a system during operation, in some systems the presence of particulate contaminates on parts during assembly can substantially contribute to particulate accumulation in the field and the resultant risk of poor performance and/or reduced component operating life. Moreover, filters are always specified with effectiveness, which means that a small percentage of undesirable particles will always pass through.
A variety of approaches to determine surface cleanliness are known and used for various components and surfaces. However, none of those approaches deal with directly measuring the presence and amount of particulate on the surface of a large component during factory assembly. Instead, these techniques focus either on measurement of particulate concentration in fluids used to wash parts or on determining the presence of organic films. The fluid concentrations are usually determined using light attenuation or refraction, often using laser beams. The organic films are determined by creating a water film on the surface and determining the formation of droplets or breakup of fluid films into rivulets, or the organic film thickness is determined by refraction of light shined at the film. The fluid based approaches cannot be adapted for practical, economic use on a mechanical assembly floor and the second mentioned technique is not relevant since the particles, not an organic film, is at issue.
In another known approach, surface replicas are used. With such replicas, the surface to be sampled is covered with either an adhesive tape or is covered with a curable material, which subsequently replicates the surface topography while also capturing loosely held surface particulate contamination. The surface is then scanned manually or with sophisticated optical recognition software to count numbers of particles and sizes. This approach is indeed used scientifically but it cannot give instantaneous results on the factory floor and in most cases would be considered prohibitively expensive. Moreover, the sample area is of necessity, exactly equal to the size of the removed sample. No amplification can be obtained this way. In other words, it is not possible to sample a larger area than that of the sampling device itself.
The invention provides a special procedure using a set of specialized equipment to quantify the particulate contamination of a component surface. In a preferred embodiment of the invention, the contamination can be amplified to provide for a more accurate reading, by sampling a larger component area then the area of the sampling device itself. With the sampling devices of the invention, a measurement is provided that has visual meaning to the operator making it and provides a convenient archival form of the contamination sample for later process audit, if necessary or desirable.
According to the well-known white glove test, an inspector wears a bright white glove and wipes a glove finger or hand across the target surface for some distance and then observes the glove where it touched the surface. The inspector then makes an arbitrary decision about whether the removed contamination is sufficiently low as to not require a recleaning of the surface. The invention provides equipment and procedures to quantify this conventional white glove test. In an embodiment of the invention, a bright white cloth swatch is rubbed on a known clean surface similar in surface roughness to the surface that is to be sampled. The surface wipe is preferably accomplished by securing the swatch to a tool adapted to be grasped by the operation and swiped on a target surface area with a generally repeatable amount of pressure. Most preferable the target area is defined, and limited by a mask. The effectively still clean swatch or smear is then measured for its reflectivity (brightness) using a reflectivity instrument. The smear is then rubbed on the surface to be tested to collect particulate contamination and the reflectivity is again measured. The loss of reflectivity is then related, for example, through imperical calibration, to the amount of contamination that was transferred to the smear in the second rubbing.
In another embodiment, a reflectivity instrument is calibrated based on the reflectivity of the clean surface rubbed swatch so that only a single rub of each swatch thereafter, on the part to be tested, is required.
In yet a further embodiment of the invention, a contamination key is provided illustrating the appearance of the cloth wipe or swatch exhibiting different amounts of particulate contamination so that following a test swipe of the component, the swatch can be compared to the key to visually approximate the level of contamination.